1. Field of the Invention
The present invention relates to a test method for testing the data storage characteristics of a memory such as a non-volatile memory, enabling secure detection of presence of a memory cell with defective memory storage ability.
2. Related Background Art
At first there will be explained, with reference to FIGS. 4 through 8, the conventional method for testing the data storage characteristics of a memory. FIG. 4 is a flow chart of the test process for the data storage characteristics of a non-volatile memory 1 (hereinafter called a "memory cell").
At first, in a step S1 for writing all bit data into the memory, a gate voltage V.sub.G is elevated by a voltage elevation circuit applied to a gate terminal 2 of each memory cell 1. In this state, a source terminal 3 is maintained at 0 V, while a drain terminal 4 is left open. FIG. 8 shows a circuit for applying a voltage to the gate terminal 2 at the data writing. In this state, a rated voltage V.sub.1 of the memory cell 1 is applied as the gate voltage V.sub.G to the gate terminal 2, whereby a predetermined amount of negative charge (-) is accumulated in a floating gate 5.
Then, in a step S2, each memory cell with the accumulated charge is left to stand in a high-temperature environment for a predetermined period. In this manner the loss of the charge, accumulated in the floating gate 5 shown in FIG. 6, is accelerated and the change in the charge amount in a prolonged period can be estimated.
Then a step S3 executes the data read-out of the memory cell 1 at a voltage V.sub.READ. FIG. 7 shows the state of the memory cell 1 at the data reading. The data reading is achieved by detecting the drain current I.sub.D at a drain terminal 4.
FIG. 5 shows the change in the V.sub.G -I.sub.D characteristics before and after the high-temperature standing test, wherein a curve 10 indicates the behavior of the drain current I.sub.D under the application of the rated voltage V.sub.1 at the gate voltage V.sub.G before the high-temperature standing test, while curves 11, 12 indicates the drain currents I.sub.D after the test.
The curve 11 shows an example of the memory cell with good data storage characteristics. As the charge accumulated in the floating gate 5 is scarcely lost, this curve is not much displaced from the position of the curve 10. Namely, this curve 11, though showing a certain charge loss, provides a normal data as it is not lowered below the evaluation reference value V.sub.READ.
On the other hand, the curve 12 shows an example of the memory cell with defective data storage characteristics (defective memory cell). The curve is significantly displaced from the position of the curve 10, because the loss of the accumulated charge exceeds a predetermined amount. Consequently, it is possible to discriminate whether the memory cell is satisfactory or defective, by evaluating the change of the drain current I.sub.D, utilizing the voltage V.sub.READ as the evaluation reference value.
However, in such conventional test method, as the data writing in the step S1 is executed by the application of the rated voltage V.sub.1 as the gate voltage V.sub.G to the gate terminal 2, a large amount of charge is accumulated in the floating gate 5 of the memory cell 1. With such large amount of accumulated charge, even if the position of the V.sub.G -I.sub.D characteristics curve is displaced, the threshold of the V.sub.G -I.sub.D characteristics may not move below the reference voltage V.sub.READ used for discriminating the data read-out in certain memory cells, so that the detection of the defective memory cells may become impossible.
Also if the high-temperature standing is extended until the curve of the V.sub.G -I.sub.D characteristics comes below the position of the voltage V.sub.READ in order to securely detect the defective memory cell, there is required a considerably long time, leading to a low work efficiency.